All of the earthquakes were between magnitude 2 and magnitude 5, and no significant damage was reported. The shaking, however, is part of a troubling recent phenomenon. The disposal of wastewater from hydraulic fracturing, the process also known as fracking, appears to have spiked the likelihood of earthquakes in Oklahoma, potentially raising the state to the same earthquake threat level as California, according to a recent USGS forecast.

Until recently, earthquakes in Oklahoma were few and far between. In 2010, the state experienced just 41 tremors. By comparison, each year Southern California alone has about 10,000 earthquakes.

The rise in earthquakes, in other words, can be attributed to the injection of large quantities of wastewater into wells deep below the Earth's surface. According to USGS, most of the underground wastewater comes from oil and gas operations — it is created when clean water mixes with dirt, metals, and other toxins below the Earth's surface during extraction operations.

The contaminated water becomes too dangerous to dump anywhere, since it could seep into regular groundwater, so companies shoot the wastewater deep into the ground, between layers of hard rock. That buried water can fracture and move previously stable rock, causing earthquakes under certain circumstances.

Much (but not all) of that wastewater injection is associated with the fracking boom, which has led the practice to become more common in recent years, especially in Oklahoma. Though the state isn't the only one experiencing a spike in wastewater injections, Oklahoma is full of eons-old fault lines that went quiet long ago. Wastewater operations seem to be shaking some of those faults loose, making the land especially vulnerable to earthquakes.

The Environmental Protection Agency's administrator, Scott Pruitt, previously served as Oklahoma's attorney general, and he has come under fire from some in the state for failing to take action against wastewater injection and fracking.